This invention relates generally to a resistor capacitor circuit.
Resistor-capacitor (RC) circuits may be utilized to form RC filters. Conventionally, an integrated RC circuit is constructed as a dedicated N-well resistor coupled to a transistor. Capacitance is provided by the transistor""s gate to substrate capacitance. The transistor may be formed in a P-type substrate having two N-type doped regions on either side of a gate situated over the substrate. A separate resistor is formed in an N-well in the same substrate. A pair of doped regions are defined in the N-well to act as the low resistance contacts of the resistor. The region between the doped regions provides the resistance. One of the resistor""s doped regions in the N-well is coupled to one of the capacitor""s doped regions in the P-type substrate.
The N-well length and the gate area may be properly sized to achieve the desired filter cut off frequency. While such circuits may work well, they use a significant amount of chip area. Thus, a relatively large amount of the silicon substrate may be taken up by these relatively simple circuits.
The conventional integrated RC circuits may also experience one or more of the following drawbacks. A metal interconnection is used between the capacitor and the resistor. This also takes additional room and adds fabrication complexity. Moreover, the capacitance and the resistance created by such a structure is lumped or concentrated at one location. Also, the RC product changes in response to input voltage changes. An increase or decrease in the input voltage changes the RC product.
Thus, there is a need for better ways to implement integrated RC circuits.